Add backup-tool: dockerized incremental backup with monitoring and Telegram alerts

[Copilot]

Homelab backup tool — dockerized, runs on swarm manager as a normal compose stack. Supports SSH hosts (rsync) and Docker container volumes (docker cp), with interactive config, retention policies, drive replication, and Telegram alerting.

Package structure (packages/backup-tool/)

• backup-tool — main entry point: continuous backup loop with background monitor, or single-shot modes (once, monitor, retention, replicate)
• setup — interactive wizard: select SSH hosts or Docker nodes, pick paths/volumes, choose drives for replication, configure Telegram bot
• Dockerfile / docker-compose.yml — privileged container, host network, mounts docker socket + SSH keys + backup destination

Library modules (lib/)

• backup.sh — rsync -a --delete --link-dest for incremental SSH backups; temp container + docker cp for volume backups
• retention.sh — per-host/path time-based (retention_days) and size-based (retention_max_size) cleanup, always keeps at least one snapshot
• replication.sh — copies latest important snapshots to all configured drives, compresses replicas via tar -czf (main copy stays uncompressed for rsync)
• monitor.sh — background loop: SSH host reachability every 30s, disk space every 60s, independent of backup schedule
• telegram.sh — stateful alerts with exponential back-off (2min → 4min → … → 8h cap), auto-resolves with recovery message
• common.sh — INI config read/write (cfg_get/cfg_set), logging, path helpers

Config layout

config/
  backup.conf          # backup_dest, interval, important_drives/folders, telegram creds
  hosts.d/
    myhost.conf        # type=ssh|docker, address, paths, retention_days, retention_max_size

Backup directory structure

/backup/myhost/var_data/
  20250101-120000/     # full snapshot
  20250102-120000/     # incremental (hardlinked via --link-dest)
  latest -> 20250102-120000

/mnt/usb1/backup-replicas/
  myhost_var_data.tar.gz   # compressed replica

Usage

docker compose run --rm backup setup   # interactive config
docker compose up -d                    # start service

README includes full Telegram bot setup instructions (BotFather → token → chat ID → test message).